Generally, washing machines are apparatuses which wash laundry using a frictional force between the laundry and water and can be classified into front-loading type washing machines and top-loading type washing machines.
In front-loading type washing machines, washing is performed using a drop of laundry while a rotating tub which accommodates the laundry rotates. In top-loading type washing machines, a rotating tub in which laundry is accommodated and a pulsator which generates a water current at a bottom of the rotating tub are provided together and washing is performed using the water current generated by the pulsator.
Also, in all front-loading type washing machines and top-loading type washing machines, laundry is spin-dried using a centrifugal force generated by rotation of a rotating tub.
As described above, washing machines operate using rotation of a rotating tub or a pulsator and generally use a motor as a device for providing torque to the rotating tub or pulsator.
As described above, motors generally used in washing machines can be classified into control type motors, so-called servo motors, which precisely control a rotary speed of a motor and a non-control type motors which do not control a rotary speed of a motor.
Control type motors each include a speed sensor which detects a rotary speed of a motor and a current sensor which detects a driving current of the motor and precisely control the driving current depending on the detected rotary speed of the motor. Control type motors described above can each precisely control the rotary speed of the motor regardless of load.
On the contrary, in general, non-control type motors each merely control rotation of a motor through a turn-on time in which power is supplied to the motor and a turn-off time in which power supply to the motor is cut off. Non-control type motors described above are relatively low-priced.
When a washing machine includes a non-control type motor, it is difficult to precisely a rotary speed of the motor. Thus, a resonance phenomenon can continuously occur during a spin-drying operation. Here, the resonance phenomenon means a phenomenon in which a vibration frequency of a rotating tub coincides with a rotation frequency caused by the motor in the spin-drying operation and thus a rotating tub violently vibrates.
In general washing machines using a non-control type motor, since rotation of a rotating tub is controlled only using a turn-on time and a turn-off time of a motor, it is difficult for a rotary speed of the rotating tub to be deviated from a resonance speed of the rotating tub.